1. Field of the Invention
The present invention relates to power supply controllers for providing a power output; in response to a variable input voltage.
2. History of the Prior
It is known in the art to provide power supply controllers for providing a power output in response to a variable input voltage. Typically, such controllers include a comparator for comparing the input voltage with a threshold value together with circuitry for driving a lead circuit when the input voltage exceeds the threshold value. During certain operating conditions of the power supply controller, such as during the startup or "sleep" mode of operation, the startup current of the controller assumes a low level so that little power is consumed. At the same time, the lead circuit must be driven to such an extent that minimum functions of the controller are provided. Among other things, the lead circuit must be maintained in a pulldown mode so as to be capable of turning off an external MOSFET or other load having capacitance in an integrated circuit portion of the controller. Otherwise, the gate on the MOSFET may become isolated or floating when the remainder of the circuit is powered down, leaving the MOSFET potentially operative. Therefore, it is necessary to ensure that the voltage on the gate of the MOSFET or other external capacitive load is discharged.
With traditional power supply controllers of the type described, the controller is shut down when the input voltage is too low. Thus, when the comparator determines that the input voltage is less than the threshold value, then the controller is shut down. At the same time, a fairly substantial startup current on the order of 500 .mu.amperes or greater must typically be maintained during the startup mode, so that the controller can function properly.
A number of attempts have been made to improve the operation of power supply controllers during the startup mode. One such technique utilizes high value resisters for all startup functions and constantly powers a low voltage regulator to bias some of the functions. Such circuits, however, are wasteful in terms of the startup current which must be present as well as having other disadvantages. Such circuits typically have a single output point and have startup current temperature dependence. The constantly powered regulator provides a continuous reference, but at the expense of the high current resulting from the substantial current drain of the high value resistors.
It would therefore be desirable to provide an improved power supply controller which consumes very little current in the startup mode when the input voltage is below a threshold value. At the same time, such controller should be capable of powering necessary functions during the standby mode while maintaining the output in a pulldown mode with no current drain penalty. Such controller should also minimize temperature dependence of the startup current and provide multiple output points.